Everyone Else is A Returnee

Story:

They forgot to take him to a picnic in elementary school. They forgot about him on the trip to the camp, in high school. He was forgotten on the high school trip. He finally became a university student, so what? Now he was forgotten by all mankind? Yu Il Han, who is left alone on Earth after all of humanity is sent to other worlds for training. His story will begin when humanity returns and the Great Cataclysm happens!